1. Field of the Invention
This invention relates to an improved rosin ester composition consisting essentially of an adduct of rosin and either or both trimethylolpropane (TMP) and trimethylolethane (TME) and to an improved hot melt adhesive composition containing said rosin ester. The present invention is particularly directed to a thermally stable, rosin ester composition containing a particular combination of antioxidants and to hot melt adhesives containing said stable rosin ester.
2. Description of Related Art
Based upon their mode of application, adhesives typically can be divided into four generic categories: solution adhesives, dispersion adhesives, emulsion adhesives and hot melt adhesives. Recently, the popularity of hot melt adhesives has increased due to their ease of application to substrates and particularly to the lower amount of pollution associated with their use. Hot melt adhesives themselves can be divided into two classes, those that are tacky even at ambient temperature, e.g., at room temperature (i.e., about 20.degree. to 25.degree. C.), and those that are tack-free at ambient conditions. The former class of hot melt adhesives are commonly referred to as pressure-sensitive, hot melt adhesives, while the latter are simply referred to as hot melt adhesives. Except when it is otherwise obvious from the context, in the present application and claims the phrase "hot melt adhesive" will be used in the generic sense to describe both those compositions that are tacky at ambient conditions, especially at room temperature, and those that are tack-free.
Hot melt adhesives are generally thermoplastic materials which contain as essential constituents a thermoplastic polymer and a tackifier. Whether a particular hot melt adhesive composition is pressure sensitive or tack free at room temperature is determined in large part by the thermoplastic polymer present in the composition, by the tackifier component and the relative proportion of each. Tack-free hot melt adhesives also generally contain a wax.
Hot melt adhesives normally are homogeneous mixtures, and are manufactured by compounding, i.e., melt blending, the thermoplastic polymer with the tackifier and one or more waxes (in the case of tack-free compositions), antioxidants and/or other stabilizing additives. The molten mixture then is coated on a substrate, especially in the case of pressure sensitive hot melt adhesives or is cooled to form rods, pellets, bricks or the like for subsequent use. In the latter case, the hot melt adhesive is used by heating the tack-free solid adhesive until it becomes molten and the molten composition then is applied to the material for bonding by pumping or pressure extrusion through nozzles or slot dies. Such adhesives quickly melt upon heating and then set to a firm bond on cooling. Hot melt adhesives often may be held in the molten form prior to use at a temperature of between about 120.degree. to 250.degree. C. for up to 24 hours or more.
Since hot melt adhesives may be maintained for extended periods at elevated temperatures, a major concern has always been the thermal stability of the adhesive composition. Two properties of hot melt adhesives that commonly exhibit thermally-induced degradation from prolonged exposure are color and viscosity. Since both of these properties directly impact on the utility of such compositions in many applications, the art has continued to look for hot melt adhesives having improved thermal stability.
As used throughout the application and claims, thermal stability is a measurement of the oxidation resistance of a composition determined by holding 100 to 200 grams of a molten sample at 175.degree. C. (350.degree. F.) for a period of about 24 hours in the case of a tackifier composition, and about 96 hours for an adhesive composition, while monitoring changes in color and viscosity. Color is measured on the Gardner scale, which assigns a numerical value between 1 and 18 to quantify the color of the sample, preferably using a Gardner Delta Color comparator. A lower Gardner value indicates a lighter color and accordingly a lower degree of oxidative degradation. Viscosity changes are measured using a Brookfield thermocell viscometer. A qualitative assessment of thermal stability also is made by visually observing any surface skinning and/or charring of the molten sample.
Rosin esters, i.e., the reaction products of a rosin acid and a polyhydric alcohol, have been used widely as tackifiers for hot melt adhesives and more generally as modifiers for rubbers. When used as a tackifier in ethylene vinyl acetate (EVA) based hot melt adhesives, such tackifiers are claimed to produce hot melts of improved clarity and viscosity stability. The preferred tackifiers for hot melt adhesives have been the glycerol and pentaerythritol esters of rosin. Numerous patents have issued directed to methods for preparing such esters, and adhesive compositions containing them, having improved thermal stability. In this regard, please refer to U.S. Pat. Nos. 3,485,783; 3,780,013, 3,869,416; 4,172,070; 4,248,770; 4,283,317; 4,302,371; 4,380,513; 4,548,746; 4,643,848; 4,650,607; 4,659,514; 4,725,384; and 4,775,497. U.S. Pat. No. 4,725,384, for example, describes two commercial EVA-based hot melt adhesive formulations, one of low polarity and one of high polarity. Both formulations include a tall oil rosin-pentaerythritol ester as the tackifier. Nonetheless, problems of poor oxygen stability and color formation continue to confront hot melt adhesive compositions containing rosin ester tackifiers.
The present invention is based on the discovery that rosin ester tackifiers comprising the adduct of rosin and a polyhydric alcohol selected from the group consisting of trimethylolpropane (TMP), trimethylolethane (TME), and their mixtures and adhesive compositions containing such rosin esters exhibit enhanced thermal stability, i.e. an improved oxidation resistance, relative to similarly prepared rosin esters and similarly compounded adhesives containing such rosin ester tackifiers based on the more commonly used polyhydric alcohols such as glycerine (glycerol) and pentraerythritol.